Small doubt regarding field theory

[shiva kumar]

hi i am shiva kumar
well i have a small doubt regarding field theory.It is said that circuit theory fails at higher frequencies ,field theory comes into picture.At the same time some devices radiate signals if not properly designed,why like that?i fail to understand this concept.

capacitive reactance Xc=1/(wc)
at lower frequencies Xc is not negligible how can you neglect it?


please try to help me understand this concept.

well what i mean why are we neglecting "impedance" at higher frequencies and con?sider only resistance?

[reachrkata]

Capacitances are usually parallel in any system (unless explicitly mounted in series).
For Eg-In a co-axial cable, there is is a capacitance between the centre conductor and the shield, in a PCB there is a capacitance between 2 traces, etc.

Secondly, in such systems, the parallel capacitance has a low pass filtering effect, i.e., high frequency signals tend to get dampened and low frequency signals are unaffected by the capacitances in the system.

Hence Xc is neglected at low frequencies.

-Karthik

[shiva kumar]

thanks for the reply karthik
but why they act like antennas i mean why the devices tend to radiate high frequencies ,has it got something to do about capacitors and inductors being energy storing elements? but why radiate?